Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates to methods and devices for treating body tissues such as tumors or lesions with thermal energy, and in particular, to such methods and devices that deploy thermally conductive elements to treat a predetermined shape of tissue.
2. Brief Description of the Related Art
Within the last ten years, interstitial thermal therapy of tumors has become an accepted method for treating cancerous tumors. These minimally invasive therapeutic procedures are used to kill cancer tumors without damaging healthy tissues surrounding it. Increasing the temperature of the tumor above a threshold level of about 70-130 C will cause tumor death. Interstitial thermal devices for thermal tissue ablation including radio frequency ablation (RFA), microwave and laser based technologies have been developed and have received 510K FDA clearance. All of these techniques use radiation to transfer the energy to the tumor, and therefore the heat in the tumor is generated indirectly through local energy absorption sites (e.g., blood in the case of a laser or fat in the case of RFA) could result in a non-homogenous heating of the tumor. The consequences of a non-uniform heating of the tumor could include incomplete death of the tumor and/or skin burns and injury of healthy tissues or organs. Incomplete tumor death will result in recurrence of multiple small tumors in the treated area.
Moreover, as most of the heat is transfer by radiation (in laser, RFA and microwave), it is very difficult to calculate the temperature distribution without precisely knowing the fine microstructure (down to the cell level) that cannot be predetermined with a non-invasive method. In addition the temperature measurements are also challenging; in these cases, since the probes could be directly heated by the energy sources and will show it""s own temperature rather than that of the tissue. For example, in laser or RFA thermocouples may get hot from the source much quicker than tissue (as they absorb RF and laser energy more than tissue) and will show temperatures that are higher than the actual temperature in the lesion. That could result in insufficient heating and if the operator increases the amount of energy delivered to the tumor, an overheating may occur which will result in burning. Another limitation of RFA is that it is not MRI-compatible.
The limitations of the prior art are overcome by the present invention as described below.
The present invention is an alternative to Laser Interstitial Thermal Therapy (LLIT) and RFA, which is used to destroy tumors or lesions through the absorption of radiation by tissue. However, as discussed above, in the LLIT and RFA processes, the temperature cannot be predicted or easily controlled due to the varying light and RF energy absorption properties of different types of tissue. In addition, RFA will interfere with implants (such as pacemakers) and the patient with such implants cannot be treated with RFA.
The present invention also destroys tumors thermally, but the heat is generated directly by electrical resistance heating conducted to the tissue rather than through the absorption of non-ionized radiation by the tissue. The process of the present invention involves computerized scanning (CAT, CT, PET, or MRI) to mathematically determine the location and shape of the tumor. The information derived from the scan allows a stereotactic frame or other technique such as ultrasound to be used to position a probe within the tumor.
The probe comprises a thermally conductive tip containing an electrical resistance heating element. The thermally conductive tip is mounted on the end of a fiber which is separated from the tip by a heat sink to avoid thermal conduction down the fiber. The fiber contains the electrical power leads and other electrical leads connecting to monitoring devices associated with the tip. The tip is coated with a thin biocompatible coating, such as diamond-like coating, ceramic, polymers and the like, to avoid coagulated tissue sticking to the tip.
The area of tissue treated by the tip is determined by the addition of one or more thin, thermal conductive elements, which may be formed of shape memory material, such as nitinol. The shape memory elements are desirably in the form of thin wires or pins which are folded against the tip at lower temperatures and which deploy at selected higher temperatures. The shape memory elements may be deployed in multiple stages at succesively higher temperatures so that succesive layers of the tumor are exposed to specific temperatures during treatment. Coagulating the tumor in successive layers is desirable to avoid hemoraging. By selecting the number, size and placement of the shape memory elements, tumors of varying sizes and shapes may be treated in a predictable, controllable fashion.
In order to control the process, the tip may also be provided with a miniature thermocouple or the like to provide temperature feedback information to control the temperature of the tip. Through knowledge of the shape and location of the tumor obtained from computerized imaging, the design of the tip and shape memory elements, and the temperature feedback information, information can be presented to the operator showing the specific progress of the treatment of a tumor and allowing predictable control of the process.
In alternative embodiments, deployable pivoted razorblades rather than thin wires are employed to conduct the thermal energy to the tumor. The razorblades are deployed mechanically rather than being deployed due to temperature dependent shape memory effects. In one embodiment, a linear actuator, comprising a threaded shaft operated by a motor, deploys the razorblade thermal conductive elements. In another embodiment, a nitinol spring is heated so as to extend and deploy the razorblade elements.
In all embodiments, a pyrolytic graphite element may be used to provide the heat source. Pyrolytic graphite has unique thermal properties in that it acts as a resistor axially but is conductive radially.
In a further embodiment, the deployable razorblades are deployed mechanically by a spring-biased copper conductor that serves a dual functionxe2x80x94as a plunger to push deploying arms on the razorblades and also as a conductor for the power supply for the pyrolytic graphite heater element. The plunger is housed in a shaft which is coated with an electrically conductive material, for example, gold, to act as the power return or ground so as to complete the electrical circuit supplying power to the heater element. When the plunger moves forwardly to push the arms on the razorblades, it may also extend a needle which helps to hold the probe in place when the razorblades deploy.
The deployable razorblades may be deployed in stages to treat the tumor layer by layer. The deployment may be triggered at specified temperatures as measured by temperature feedback elements in the probe tip.
The present invention uses thermal conduction, as opposed to radiation absorption, to heat the tumor/lesion volume. Since the thermal properties of tissue are relatively homogenous, the results can be predicted. The shape of the probe tip in the form of the deployable thermal conductive elements may be altered during treatment. The combination of shape and activation temperature can be predetermined for any specific tumor/lesion geometry. This offers the following advantages: highly predictable temperature distribution; larger areas can be effectively treated, in a controlled manner, since the heat is dissipated primarily by conduction; localized carbonization will not result in tunneling and the process is safer than LLIT or RFA; the maximum temperature in the treatment zone will never exceed the temperature at the tip of the probe, and therefore, one can easily control the maximum temperature within the tumor/lesion and adjacent tissues; temperature may be actively controlled via closed loop feedback system, where the maximum temperatures are measured during the process by placing miniature thermocouples at the end of the thermal probe.
These and other features, objects and advantages of the present invention will become better understood from a consideration of the following detailed description of the preferred embodiments and appended claims in conjunction with the drawings as described following.